1. Field of the Invention
This invention relates to an apparatus for developing an electrostatic latent image, and more particularly to an apparatus for developing an electrostatic latent image into a toner image in which the thickness of a toner layer which is used for forming the toner image is controlled to control the density of the toner image.
2. Description of the Prior Art
In a touch down development of a donor development method of electrostatography in which an electrostatic latent image is developed into a toner image and the toner image is transferred to paper or the like or is used as a final image, the image density changes with the thickness of a toner layer which is used for forming the toner image. The thickness of the toner layer is controlled, for instance, by use of a doctor blade projection toward a developing roller. A part of the toner layer is scraped off by the edge of the doctor blade to obtain a desirable thickness of the toner layer.
The inventors of this invention have proposed an electrostatic print marking apparatus in which steel plates are printed with markings which, for example, indicate the dimensions of the steel plate, the date of manufacturing of the steel plate and other pertinent information immediately after the steel plates have been fed out of a rolling mill. In the print marking apparatus, the traveling speed of the steel plates fed out of the rolling mill is measured and the marking apparatus is driven in synchronization with the speed of the steel plates. The speed of the steel plates fed from the rolling mill is not constant, but depends on the condition of the rolling operation. Therefore, the speed of rotation of the developing roller and an electrostatic recording drum is changed according to the speed of the steel plates, which results in a change in the length of the development time and accordingly a change in image density of the toner image obtained.
In an electrostatic apparatus, it is always desirable that the toner image be developed in a constant density. Though it is possible to control the image density of the toner image by controlling the thickness of the toner layer by use of a doctor blade, in actual practice, the adjustment of the doctor blade proves to be difficult and requires a complex control system.